Process for the production of developed electrophotographic images including application of a breakdown potential to discrete small areas of a photoconductor



United States Patent 3,234,017 PROCESS FOR THE PRGDUC'HDN 0F DEVELQPED ELECTRGPHOTOGRAPHEC IMAGES INCLUDKN G APPLICATION OF A BREAKDOWN POTENTiAL T0 DISCRETE SMALL AREAS OF A PHOTO- CONDUCTDR Gerhard Heyl, Cologne-Stammheim, and Hildegard Haydn, Leverkusen, Germany, assignors to Agfa Aktiengesellschaft, Leverirusen, Germany, a corporation of Germany No Drawing. Filed Nov. 1, 1960, Ser. No. 66,497 Claims priority, application Germany, Nov. 5, 1959, A 33,197 7 Claims. (Cl. 961) This invention relates to electrophotographic processes in which an insulating, photoconducting surface is charged and, after image-wise exposure, is dyed with a fine powder or aerosol (toner). A large number of the prior known development processes produce in such cases images havin the disturbing property that only the margins are dyed when the areas to be dyed are relatively large, or the margins are more strongly dyed than the inner parts of the said areas. The cause of this phenomenon is assumed to be that the field intensity at the margins is particularly strong and thus also the force on the charged toner particles.

It has now been found that when using a photoconducting layer applied to a sufliciently conductive support, uniformly dyed images are obtained after charging, imagewise exposure and development by means of toners if the photoconducting layer is non-homogeneously charged and development is effected with toners having a large electrical polarisability which are not or only slightly charged.

in contrast to the known processes, the present process does not make use of the forces which exist between the charged toner and a like or oppositely charged surface, but the forces which operate in non-homogeneous zones on uncharged toner particles. In this case, there are re quired of both the photoconducting layers and the toners which are used, properties which are in part in strict contrast to those necessary wtih the known processes.

Processes are already known in which an originally homogeneous charge distribution is made non-homogeneous by a screen being exposed prior to, during or after the image exposure. Copying by means of a screen, however, is complicated and involves considerable time. The process according to the invention differs fundamentally from this arrangement, in that thecharging of the photoconducting layer is made non-homogeneous from the outset and an electrically uncharged toner of high polarisability is used.

A non-homogeneous charge distribution can be achieved in various ways. For example, in the Corona charging,

the charging potential can be so chosen that the disruptive field intensity is exceeded in the photoconducting layer. In the case of a layer having a thickness of 10-30; and which has been prepared in known manner from zinc oxide in silicone resin, the necessary voltages are for example about 7 to kv. The necessary voltage can also be adjusted with the aid of control electrodes known per se. In the prior known processes and in contrast to that described herein, disruptions are carefully avoided and care is taken that a uniform charging is obtained.

Due to the disruptions distributed statistically over the area, small individual, discrete surface elements are more or less discharged, but extremely rarely completely discharged; the charge distribution and thus also the field intensity through the layer becomes non-homogeneous. The charging potential necessary for the disruptions depends with each photoeonducting layer on the layer thickness. In order to produce a large number of small, par- "ice tially discharged islands, the layer thickness must be constan, uniform and free from pores. The disruptive strength of the layer can be influenced in the desired manner by incorporating finely dispersed substances therein. By suitable choice of the size, of the distribution of the incorporated substances and of the charging voltage, it is possible to produce the desired value for the islands partially discharged by the disruptions. The polarity of the charge also has an influence on the disruption process.

With negative charging, the partially discharged surface elements are usually smaller. The activity of the additives depends on their dark conductivity and the influence on the disruptive strength which is important in the present process. Where the incorporated substances are more conductive than the photoconductor in the dark, their action is based on the fact that the charge already discharges from small islands during or shortly after the charging. The necessary charging voltage can be very greatly reduced down to about 500 volts by the additives. In this case, the ions can naturally no longer be produced by a Corona discharge. The already known production of ions by means of radio-active ionisers and the deposition thereof on the electrophotographic material in a relatively weak field has proved suitable. It has been found that with the incorporation of conductive substances in concentrations higher than about 40 percent, relating to the photoconducting substance, the discharging of the charges from the individual islands takes place independently of the charging voltage. With low concentrations and/or with poor conductivity, the incorporated substances act mainly by the reduction of the disruptive strength, so that the lack of homogeneity of the charge is only formed above a certain charging voltage. The grain size of the added substances is advantageously about the grain size of the photoconductor, but can be up to about 15a.

A large number of the substances can be employed as additives. Their photoconductivity is of no importance for this process. Suitable for the incorporation are numerous and preferably uncolored organic and inorganic substances which exist in a suitable grain size and the specific resistance of which is approximately between 10' and 10 ohms-cm.

Examples of these are metal oxides, sulphides, selenides and carbonates such as zinc sulphide, zinc oxide, antimony oxide, arsenic trioxide, titanium dioxide, tin dioxide, aluminium oxide, boron trioxide, silicon dioxide, bleaching earth, talcum, kaolin, zirconium oxide, cerium oxide, beryllium oxide, strontium oxide, tin sulphide, arsenic sulphide, cadmium sulphide, sulphur, selenium, calcium carbonate, magnesium oxide, barium oxide, calcium oxide, lead sulphide, lead selenide and selenium furthermore mercuric chloride, cryolith, barium sulphate, lead acetate, powdered glass and organic substances such as anthracene, terphenyl, chrysene, phenanthrene, pyrene, fluorene, benzanthrene, acenaphthene, carbazole, naphthalene, benzoquinone, anthraquinone, diphenyl, phthalodinitrile, benzoic acid, uric acid, benzophenone, phenol, aminophenol and phthalic anhydride, hydroquinone, polyvinyl acetate, starch and powdered cellulose. It will be seen from the compounds listed that the chemical nature of the compounds are of no significance.

When the charging is effected by friction, another possibility for the production of non-homogeneous charging distributions is to make the surface of the photoconducting layer irregular. Only electrophotographic materials have so far become known in which, contrary to the material according to the invention, the support was provided with a screen or lenticulation, While the surface of the layer was flat and developed with charged toner. In the process according to the invention, the irregularities in the surface which are always present but which are usually insufiicient for the purpose envisaged are for example intentionally formed by suitable casting conditions. When a depth of 1 to 20;, has proved to be a suitable size for' the surface structure produced for example by means of an embossing cylinder. The mean spacings of the islands partially discharged by disruptions is likewise about to 1000 A coarser structure is unsuitable.

Theform of the surface structure can moreover ebe chosen to be very different.

punctiform, and both regular and irregular. Irregularly distributed semispherical or conical protuberances of different sizes have proved especially suitable.

The irregularities need not consist in unevenness or ruggedness of the surface only. Also irregularities with regard to electrical for example tribo-electrical properties of the surface of the photoconductive layer cause an inhomogeneous charging when being rubbed.

It is very important for the present process that the re-- solving power is not determinedby the structure of the charge distribution. the charging voltage, there is no complete discharge .by the disruptions. The charging density difference being set up with a disruption causes on the one hand such a large unhomogeneity of the field, that a uniform dyeing of large surfaces is effected. On the other hand, the residual charge is still sufficiently large to supply by expo- It can be both linear and For example, by suitable choice of.

sure to light a developable electrostatic contrast. The

same applies as regards the charging by friction.

The properties of the toner best suitable for the process of the invention are substantially different from those formerly required. It is a particular advantage that uncharged toner is used and thus the charging of the toner, perhaps with the aid of a support or a Corona discharge or by friction is superfluous.

duces a very sharply pronounced marginal eifect without dusting of the surfaces to be dyed. Instead of the charging of the toner, the polarisability of the toner particles is important in the process according to the invention. The. polarisability of the toner must be large, so thatthe forces on the particles in non-homogeneous fields also become large. In order that the lack of homogeneity of the charge distribution on relatively large surfaces to be uniformly dyed is not made visible by the toner, the latter mustnot be too fine. Grain sizes of about 5 to 20p have proved to be especially desirable.

The polarisability of the toner is achieved according to the invention by one or more of the following steps. Substances with a large dielectric constant of at least 57 are per se suitable as toner material for example barium titanate or titanium dioxide, but conductivity which must be greater than that of the photoconductive substance, that is greater than 10 0hl'11ST -Cm. of the toner or the toner composition is better. This conductivity can be achieved by a conductive toner substance for example carbon black, lamp black, graphite, charcoal (from wood) or conductive organic pigments of different colors such as aniline-black (Helioschwarz TW, tradename of Farbenfabriken Bayer AG) or azopigments (ID-Schwarz, tradename of Farbenfabriken Bayer AG) and also by additives to a toner substance which per se is insulating. Moreover, the toner particles which per se are insulating such as carbon black dispersed ina thermoplastic resin can be enclosed by a conductive sheath.. Known resinsv of low melting point for example bitumen, polyethylene, poly- With homogeneous charg ing, a toner suitable for the process described herein pro-= 4 methacrylate or the like which are dyed with carbon black or other known pigments and improving agents are suitable as toner substance. additives or as conductive preparations for the surfaces are the substances'which are known per se for antistatic preparation, such as for example: sulphuratedoils, alkyl i sulphonates, longrchain alcohols ethe'rs or esters, phosphorici. acid esters, polymethaorylic acid, polyethylene oxide derivatives.

Since the polarisability of a substance. isinot only. de-.-

termined by the dielectric constant .and the conductivity but also by the. volume and the shape of the particle; of

said compound, the force in inhomogeneous fieldscan be Rod-like. particles are attracted with very much greater force in: non-homogeneous 'fields than spherical particles on acintensified by the shape of the; toner. particles.

count of the smaller de-electrifying factor. For examplc the attractive power of a particle having a dielectric constant of 7 with a proportionof the, length and the. thickness like 10:1 is: three times as great as that for spherical particles.

The development is moreoverv effected by any one of the prior known processes which preferentially provide the marginal effect, foriexample by the toner being scat- 'tered on the layer and thereaftershaken or blown off the said layer; Since thealack of-homogeneity of the field is necessary, this being in contrast-to the prior known processcs, no development electrodes need be used just above the layer when developing with clouds of toning agent."

Unavoidable charging of a toning agent, regardless of sign, has no influence, as long as the chargingdoesnot exceed a predetermined jlimit and/orthe toner particles ticles are larger. 1y of a possibly existing charging in thenon-homogeneous field of the charged areas, a positive image is formed. The. process is suitable for both line copie's with large areas to be colored and alsofor half-tone images. It can be used with electrophotographic material in which the photoconducting layer is applied to a metallic support, or in which the photocondu'cting substance is applied in a binder to a sufliciently conductive support;

The process will now be more fully explained by the following example.

A. Photoconductive layer 10 parts, by .weightof siliconeresin, for. example Bayer- P K 60 percent in toluene 10 parts by weight of zinc oxide PA Merck I .5 parts by weight of calcium sulphate 20 parts by weight of toluene 1 are crushed in a ball mill.v

This solutionis applied as a layer: to paper by methods known per se..

B: Toner:

A normal-commercial toner, Graph-O-Fax No. 2 (bitumen and graphite), Philips A. Hunt Company, is stirred with addition of a wetting agent:.into a solution of 20 g.

of Tallopol GK..(phosphoric acid ester), Stockhausen,

per litre of water. The: toner is filtered oif, dried at 55 C. in a drying. chamber, crushed in a mortar and screened.

C. Charging; development and fixing- The charging is effected .with a Corona-discharge, .discharge :voltage 6 kv., spacing :between discharge wires 5 and layer, 2 cm. After exposure, thetoner is scattered on the layer and the image. isdeveloped by moving the toner backwards and forwardsovert the exposed surface.

To be considered as conductive Fixing is obtained by heating the developed image for fusing the toner or by treating with organic solvents.

In the above example three parts by weight of glass powder or 5 parts by weight of calcium carbonate can be used instead of 5 parts by weight of calcium sulphate.

The charging can also be produced with Perlon or Dralon velvet instead of a Corona discharge.

If no additives are used in the layer, a sufficiently nonhomogeneous charging with uneven layers is certainly produced by friction, but not by a Corona discharge at voltages below about 5 kv.

For the preparation of the toner, it is also possible to use a dispersion of 2 g. of stearic acid ester in 1 litre of water. In both cases, with homogeneous charging of the toner, only a narrow and sharp coloring of the margins is obtained, whereas unexposed and exposed areas remain white.

It will be clear to those skilled in this art that the practice of the invention lends itself readily to a number of useful modifications in method, apparatus, for example charging devices, electrophotographic materials and toners. For example the electrophotographic material is not limited to the previously mentioned because it may comprise any suitable compositions, for example the photoconductive layer may be a homogeneous one consisting of selenium, sulphur or organic photoconductive products like anthracene or mixtures thereof. Furthermore, the photoconductive layer may consist of a dispersion of solid photoconductors known per se in insulating binding agents such as silicone resins or any suitable organic resins for example on the base of polyurethanes, polyesters, polycarbonates, polystyrene, chlorinated rubber, acrylic resin, vinylchloride-acetate resin or the like. Furthermore, the photoconductive substance such as organic photoconductors may be solved in the binding agent forming another type of homogeneous photoconductive layers. Also the support being transparent or not on which the photoconductive layer is applied may be paper or any film-like material consisting of film-forming agents having a sufficient conductivity. Furthermore, the fixing may be accomplished by fusing the toner or by treating the developed electrophotographic image with organic solvents or vapours of organic solvents, like methylene chloride, trichloroethylene and the like.

What is claimed is:

1. In an electrophotographic process which comprises applying an electrostatic charge to a continuous layer of photoconductive insulating material, projecting a light image onto the said layer whereby a flow of electricity takes place through said layer producing an electrostatic latent image at a surface thereof, and developing the said image by depositing toner particles on the said surface to which the said toner particles adhere in a distribution varying in density with the intensity of the electrostatic image charge at the various parts of the surface, the improvement which comprises initially applying the electrostatic charge to the layer of photoconductive insulating material by subjecting it to an electric potential by means of a Corona discharge, which potential is large enough to cause the layer to break down electrically at a multiplicity of closely spaced discrete locations and to charge the surface portions between the broken down sites, and depositing toner particles which are essentially uncharged.

2. The process of claim 1 in which the layer of photoconductive material has its breakdown strength reduced by the incorporation therein of a finely divided filler having a specific resistance between about 10 and 10 ohmcentimeters.

3. An electrophotographic process as defined in claim 1 in which the toner particles that are used have a dielectric constant of at least 5.

4. An electrophotographic process as defined in claim 1 in which the toner particles that are used have an electrical conductivity that is higher than that of the photoconductive insulating layer.

5. An electrophotographic process as defined in claim 1 in which the toner particles that are used have rod-like configurations.

6. The process of claim 1 in Which the charged islands have mean spacings of no more than 1000 microns from each other.

7. The process of claim 4 in which the toner particles are coated with an antistatic agent.

References Cited by the Examiner UNITED STATES PATENTS 2,297,691 10/ 1942 Carlson 96-1 2,598,732 6/1952 Walkup 9645 2,599,542 6/1952 Carlson.

2,777,957 l/1957 Walkup.

2,853,383 9/1958 Keck 961 2,917,385 12/1959 Byrne 96-1 2,965,573 12/1960 Gundlach 252-62.1 3,005,707 10/ 1961 Kallamann et a1.

3,080,251 3/1963 Claus 25.2-62.1

OTHER REFERENCES Perry, Chemical Engineers Handbook Third Edition, 1950, McGraw-Hill Publ. Co., page 1732 relied on.

Sears et al., University Physics, Addison-Wesley Publ. Co., 1955, 2nd Ed., Chapters 25 and 27 relied on.

NORMAN G. TORCHIN, Primary Examiner.

PHILIP E. MANGAN, Examiner. 

1. IN AN ELECTROPHOTOGRAPHIC PROCESS WHICH COMPRISES APPLYING AN ELECTROSTATIC CHARGE TO A CONTINUOUS LAYER OF PHOTOCONDUCTIVE INSULATING MATERIAL PROJECTING A LIGHT IMAGE ONTO THE SAID LAYER WHEREBY A FLOW OF ELECTRICITY TAKES PLACE THROUGH SAID LAYER PRODUCING AN ELECTROSTATIC LATENT IMAGE AT A SURFACE THEREOF, AND DEVELOPING THE SAID IMAGE BY DEPOSITING TONER PARTICLES ON THE SAID SURFACE TO WHICH THE SAID TONER PARTICLES ADHERE IN A DISTRIBUTION VARYING IN DENSITY WITH THE INTENSITY OF THE ELECTROSTATIC IMAGE CHARGE AT THE VARIOUS PARTS OF THE SURFACE, THE IMPROVEMENT WHICH COMPRISES INITIALLY APPLYING THE ELECTROSTATIC CHARGE TO THE LAYER OF PHOTOCONDUCTIVE INSULATING MATERIAL BY SUBJECTING IT TO AN ELECTRIC POTENTIAL BY MEANS OF A CORONA DISCHARGE, WHICH POTENTIAL IS LARGE ENOUGH TO CAUSE THE LAYER TO BREAK DOWN ELECTRICALY AT A MULTIPLICITY OF CLOSELY SPACED DISCRETE LOCATIONS AND TO CHARGE THE SURFACE PORTIONS BETWEEN THE BROKEN DOWN SITES, AND DEPOSITING TONER PARTICLES WHICH ARE ESSENTIALLY UNCHARGED. 